1. Field of the Invention
The present invention relates to a method used in a communication system and related communication device, and more particularly, to a method of handling tone map interpolation in a communication system and related communication device.
2. Description of the Prior Art
A modern high-speed communication system, e.g. asymmetric digital subscriber line (ADSL) system or power line communication (PLC) system, usually employs orthogonal frequency-division multiplexing (OFDM) or discrete multi-tone modulation (DMT) technologies for transmitting and receiving signals. The OFDM and DMT systems usually allocate different number of bits of each sub-carrier to maximally utilize the channel capacity of a stationary channel. The allocation is described in a tone map. Generally speaking, the tone map can refer to the per-tone mapping of any carrier-dependent information. The design of tone maps has been a tradeoff point between channel efficiency and implementation complexity.
One factor that affects implementation complexity is the size and the amount of tone maps. The size of a tone map grows with the number of tones (which is the frequency resolution) and the bit resolution. For a stationary channel, such as the phone line in an ADSL system, one tone map is enough to describe the channel characteristics. Instead, power line is a cyclostationary channel that channel characteristics vary over AC cycles. To fully utilize the cyclostationary channel, the tone map has to be changed cyclically over time. The time resolution of tone maps adds another dimension of complexity. For a networking system over a shared medium, such as PLC, a station can have multiple links with other stations and have to manage a set of tone maps for each link. As a summary, the total size of tone maps grows with the bit resolution, the frequency resolution, the time resolution, and the number of links. High-resolution tone maps cost large tone map storage, management overhead, and communication overhead.
Further, the period of tone maps affects performance in contradicting ways for MAC and PHY if a packet is assumed to use the same tone map over time. For example, long period results in low PHY data rate, since the bit allocation must consider the worst-case signal to noise ratio (SNR) in that period. While short period brings in large transmission overhead, since a packet must be fragmented to fit in one tone map period. Short period also results in large storage overhead for storing more tone maps.
In existing standards, such as IEEE 1901/Homeplug AV, a beacon period, which may be multiple of power line cycles, can be separated into small segments, and each segment can use a different tone map. The tone map for a segment has to consider the worst SNR for each tone. The larger the segment is, the worse SNR can be seen, and the lower the allocation can be. The smaller the segment is the large storage for tone maps is needed. Further, since one packet can use only one tone map, the smaller segment means the shorter packet length, and therefore transmission efficiency is lower. Moreover, tone map resolution has been an important factor for both performance and storage. Without careful design, MAC rate even decreases with PHY rate when resolution increases.